Add fill_missing_chunks to examples of config options.

Author: williamsnell

docs/user-guide/config.md

@@ -30,6 +30,7 @@ Configuration options include the following:
 - Default Zarr format `default_zarr_version`
 - Default array order in memory `array.order`
 - Whether empty chunks are written to storage `array.write_empty_chunks`
+- Whether missing chunks are filled with the fill value on read `codec_pipeline.fill_missing_chunks` (default `True`). Set to `False` to raise a `MissingChunkError` instead.
 - Async and threading options, e.g. `async.concurrency` and `threading.max_workers`
 - Selections of implementations of codecs, codec pipelines and buffers
 - Enabling GPU support with `zarr.config.enable_gpu()`. See GPU support for more.